The invention relates to a device for performing measurements on an object comprising a core and at least one transparent layer surrounding it, said device comprising a measuring space for accommodating the part of the object to be measured, at least one radiation source unit for supplying a radiation beam with which the object is irradiated and a radiation-sensitive detection system for detecting radiation from the measuring space. The invention also relates to a method of manufacturing a fibre and to a fibre manufactured by means of such a method.